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DCCXXXIII.* 


ON  RATING  ELECTRIC-POWER  PLANTS  UPON 
HEAT-UNIT  STANDARD. 


THE 


BY  WM.  S.  ALDRICH,  MORGANTOWN,  W.  VA. 

(Member  of  the  Society.) 


The  progress  of  power-plant  engineering  has  reached  such  a 
stage  of  development  that  electric-power  plants  should  be  con 
tracted  for  on  a somewhat  similar  basis  of  guaranteed  perform- 
ance as  that  now  in  vogue  for  pumping  plants.  There  should 
be  guaranteed  a definite  output,  in  the  case  of  the  electric  plant, 
to  be  measured  at  the  switchboard  and  expressed  in  kilowa^s, 
per  1,000,000  B.  T.  U.,  supplied  to  the  steam  used  in  the  whole 
plant. 

Following  are  some  of  the  advantages  of  having  such  a stand- 
ard for  this  purpose  founded  upon  the  heat -unit  basis : 


(1)  It  is  a simple  basis , involving  quantities  easily  measured. 


(B.  T.  U.  Input.)  The  computation  is  based  upon  the  quan- 
tity of  heat  required  to  raise  all  of  the  feed  water  from  its  tem- 
perature to  that  of  the  steam  at  the  boiler  pressure,  with  such 
additional  determinations  and  allowances  as  are  now  regularly 
made  in  obtaining  the  similar  quantity  of  heat  supplied  in  the 
duty  trials  of  pumping  engines. 

(Kilowatts  Output.)  The  work  is  obtained  from  corrected 
voltmeter  and  ammeter  readings,  at  the  switchboard,  for  a defi- 
nite interval  of  time  at  a given  specified  load,  which  is  main- 
tained uniform  throughout. 


„ *To  1)e  Panted  at  tl.e  Hartford  meeting  (May,  1897)  of  the  American 
Society  of  Mechanical  Engineers,  and  forming  part  of  Volume  XVIII.  of  the 
Transactions. 


V 


X\ 


^18 


\y^' 


2 


ON  RATING  ELECTRIC-POWER  PLANTS. 


(2)  It  applies  to  all  plants  operated  by  any  hind  of  heat  engine.  1 

The  present  way  of  stating  such  performance  is  : (a)  kilowatts 
per  pound  of  coal ; (b)  kilowatts  per  cubic  foot  of  gas ; (o')  kilo- 
watts per  gallon  of  oil— according  to  the  kind  of  heat  engine 
furnishing  the  motive  power  for  the  plant.  The  proposed  stand- 
ard is  superior  to  such  ratings  as  the  latter,  and  furnishes  a 
common,  practical  and  scientific  one  for  all  of  them.  They 
involve  the  efficiency  of  the  boiler,  or  of  the  gas  producer,  and 
this  last  requires  the  determination  of  the  thermal  value  of  the 
fuel.  It  is  not  alone  the  difficulty  of  such  determinations  but 
the  unsatisfactory  nature  of  the  fuel  basis  which  has  led  to  its 
disuse  in  pumping  plants. 

(3)  It  forms  the  most  satisfactory  basis  for  comparison  of  plants. 

Efforts  are  being  continually  made  to  obtain  such  ratings  for 
existing  electric-power  plants  as  will  enable  the  probable  per- 
formance of  similar  projects  to  be  predetermined  when  installed 
and  operated  in  like  manner.  Sufficient  and  reliable  data  may 
be  obtained  at  various  proportional  parts  of  the  full  load  of  the 
plant  as  will  enable  one  to  determine  what  may  be  called  the 
characteristics  of  the  particular  type  of  electric-power  plant 
under  consideration.  The  inherent  advantages  of  each  type  of 
plant  will  then  appear  in  its  characteristic  curves,  showing  the 
variations  in  its  efficiency  and  economy  at  various  proportional 
parts  of  its  full  load  or  rated  normal  capacity.  Then  it  will 
be  possible  to  compare,  at  different  loads,  system  with  system 
and  plant  with  plant.  Such  determinations  and  comparisons 
will  be  all  the  more  valuable  if  based  upon  commonly-accepted 
standards  and  ratings,  such  as  the  heat  unit  proposed. 

(4)  It  will  facilitate  the  predeterminaticn  of  the  performance  of 

electric-power  plants. 

The  engineering  precedent  which  will  be  established  by  such 
a standard  will  promote  the  predetermination  of  the  efficient 
and  economic  performance  of  electric  plants  quite  as  much  as 
similar  ratings  have  accomplished  for  pumping  plants.  While 
both  units  are  rarely  built  by  the  same  concern,  as  in  the  case 
of  the  latter,  still  this  should  not  debar  the  installing  engineer 
from  advocating  the  use  of  such  guarantees  based  on  the  heat- 
unit  basis  if  his  work  is  to  hold  its  own  in  the  light  of  the 


ON  RATING  ELECTRIC-TOWER  I LA  NTS. 


8 


o'uarantees  and  contracts  made  in  other  branches  of  engineer- 
ing and  notably  so  in  the  installation  of  pumping  machinery. 

(5)  It  will  promote  the  most  economical  arrangement  of  plants. 

The  power  plant  is  an  aggregation  of  units  and  of  an  ever- 
increasing  complexity.  It  is  therefore  quite  as  essential  to  have 
the  whole  system  economically  arranged  as  to  have  the  most 
economical  units.  It  is  this  economy  of  arrangement  or  of 
installation,  so  to  speak,  which,  in  a measure,  the  duty  of  a 
pumping  plant  so  clearly  expresses  when  based  upon  the  heat- 
unit  standard.  It  is  such  a method  of  stating  the  final  outcome 
of  the  arrangement  of  all  of  the  details  of  a power  plant  of 
which  designers,  builders,  managers,  and  owners  wish  to  know 
the  value. 

(6)  Heat-unit  specifications  will  form  proper  basis  of  agreements. 

Builders  of  both  engines  and  dynamos  are  equally  interested 
in  the  adoption  of  some  such  common  standard.  Unfortunately, 
however,  in  many  cases,  this  interest  extends  only  so  far  as  the 
economic  performance  is  concerned  of  the  individual  machines 
which  they  manufacture. 

(7)  Contract  trials  of  electric  plants  should  be  based  on  heat  units. 

Contract  trials  of  the  completed  plant  are  necessary  to  estab- 
lish the  guarantees  of  satisfactory  fulfilment  of  contract  as  to 
both  efficiency  of  installation  and  economy  of  operation.  The 
present  method  of  basing  the  performance  of  such  upon  the 
final  plant  efficiency  is  misleading.  In  electric  installations, 
particularly,  there  is  a set  ot  conditions  insuring  a maximum 
value  for  such  an  efficiency,  usually  at  some  fractional  load. 
There  is  also  in  such  plants  another  set  of  conditions,  at  some 
other  load,  insuring  maximum  economy  of  operation  of  the 
engine.  Only  contract  trials  for  definite  periods  of  time,  at 
specified  and  uniform  loads  (at  various  proportional  parts  of  the 
full  load),  will  enable  all  claims  to  be  adjusted  regarding  guar- 
anteed efficiency  or  economy,  when  such  is  based  upon  the  quan- 
tity of  heat  supplied  to  the  system  in  thermal  units. 

(8)  It  will  advance  this  industry  along  engineering  lines. 

The  business  of  power-plant  design,  construction,  installation, 
and  management  is  not  altogether  in  a formative  period.  Nev- 


4 


ON  RATING  ELECTEIC-POWER  PLANTS. 


ertheless,  when  an  electric-power  plant  can  be  contracted  for  on 
the  basis  of  so  many  kilowatts  output  per  1,000,000  B.  T.  U.  in 
the  steam  supplied  to  the  plant,  and  that  at  a certain  specified 
load,  or  proportional  part  of  the  full  load,  then  we  may  expect 
somewhat  the  efficiencies,  economies,  guarantees  and  contracts 
now  being  regularly  realized  in  some  of  the  other  lines  of  power 
generation. 

The  subject  is  developed  in  this  paper  as  follows : 

(1)  The  Heat-Unit  as  a Basis  for  Rating  S 'earn  Power  Plants. 

(2)  The  Heat-Unit  Required  for  such  a Standard. 

(3)  Present  Use  of  the  Heat-Unit  in  Steam-Pumping  Plants. 

(4)  Present  Way  of  Stating  Performance  of  Electric  Plants. 

(5)  The  Load  Factor  in  Power-Plant  Ratings. 

(6)  Proposed  Use  of  Heat-Units  in  Electric  Plants. 

(7j  Determination  of  the  Heat  Supplied  to  the  Steam. 

(8)  Performance  of  the  Boiler  not  in  Evidence. 

(9)  Determination  of  Work  Done  by  Electric  Generators. 

(1)  THE  HEAT-UNIT  AS  A BASIS  FOR  RATING  STEAM-POWER 

PLANTS. 

It  is  not  the  purpose  of  this  paper  to  review  the  line  of  argu- 
ment for  such  a use  of  the  heat  unit  as  a standard  basis  for 
rating  and  comparing  the  performance  of  steam-power  plants  in 
general.  Nor  is  it  proposed  to  advocate  anew  the  great  value 
of  heat-unit  specifications  as  the  proper  basis  of  agreement 
between  contractor  and  builder,  on  the  one  hand,  and  the  sub- 
sequent contract  trials  on  this  basis  as  the  most  satisfactory 
means  of  adjusting  all  claims  in  power-plant  installations,  on 
the  other  hand. 

The  heat  unit  is  the  most  scientific  basis  for  the  engineer  to 
use  in  stating  the  final  performance  of  any  power  plant  driven 
by  a heat-engine.  It  is,  in  consequence,  the  most  satisfactory 
standard  upon  which  to  base  an  agreement  between  the  con- 
tracting parties  for  the  installation. 

The  merits  of  the  heat-unit  standard  in  these  particulars  * 
cannot  be  longer  open  to  discussion.  It  has  been  so  completely 
defined  for  pumping  plants,  for  instance,  and  is  in  such  constant 
and  satisfactory  use  in  this  branch  as  to  prove  its  engineering  <?• 
value. 

Heat  unit  ratings  are  also  coming  into  more  general  use  as  the 
most  suitable  standard  upon  which  to  base  the  performance  of 


ON  EATING  ELECTRIC-POWER  PLANTS. 


5 


prime  movers  in  power  plants  deriving  their  energy  from  fuel, 
whether  by  steam-engines  or  other  heat-engines.  The  value  of 
such  a standard  has  been  ably  advanced  by  Prof.  C.  H.  Pea- 
body in  a paper*  before  this  Society,  from  which  we  quote  as 
follows : 

“ It  is  customary  to  state  the  performance  of  a steam  engine 
in  pounds  of  steam  used  per  horse-power  per  hour,  a method 
which  is  open  to  objection,  since  the  value  of  a pound  of  steam 
depends  on  the  pressure  and  quality  of  the  steam.  It  has  fre- 
quently been  urged  upon  the  attention  of  engineers  that  the 
British  thermal  unit  (B.  T.  U.)  should  be  used  in  stating  the 
performance  of  engines.  ...  In  order  to  obtain  convenient 
numerical  quantities  it  is  advisable  to  state  engine  performance 
in  British  thermal  units  per  horse-power  per  minute.  Incident- 
ally, this  method  has  the  advantage  that  it  may  be  used  for  any 
heat  engine,  such  as  a hot-air  engine  or  a gas  engine.” 

(2)  THE  HEAT  UNIT  REQUIRED  FOR  SUCH  A STANDARD. 

The  acknowledged  scientific  standard  of  temperature,  of 
0 degree  Cent.,  is  evidently  impracticable  for  a heat  unit  stand- 
ard. Prof.  H.  A.  Howland  has  shown  that  such  a temperature 
is  out  of  the  question,  because  the  mechanical  equivalent  of 
heat  is  not  definitely  determined  and  probably  cannot  be  at  this 
critical  point.  Nevertheless,  such  is  the  basis  of  the  scientific 
heat-unit,  the  calorie — the  quantity  of  heat  required  to  raise  the 
temperature  of  one  kilogram  of  pure  water  one  degree  from 
0 degree  Cent. 

The  temperature  of  39.1  degrees  Fahr.,  at  which  water  attains 
its  maximum  density,  is  equally  undesirable,  notwithstanding 
that  it  is  the  old  British  standard  temperature  of  the  days  of 
Rankine.  The  determination  of  the  mechanical  equivalent  of 
heat  at  this  critical  temperature  offers  similar  difficulties  to  those 
of  the  freezing  point.  At  neither  of  these  points  can  the  specific 
heat  of  water  be  satisfactorily  determined. 

“ There  is  practical  convenience  in  choosing  62  degrees  Fahr. 
for  the  standard,  because  it  is  near  the  mean  temperature  of  the 
air  during  experimental  work,”  as  Prof.  C.  H.  Peabody  lias 
pointed  out.  Besides,  the  specific  heat  of  water  may  be  defi- 

* Transactions  of  the  Armrican  Society  of  Mechanical  Engineers,  Vol.  XIII., 
No.  484,  on  the  “Economy  and  Efficiency  of  the  Steam  Engine,’'  by  C.  H. 
Peabody. 


6 


ON  EATING  ELECTRIC-POWER  PLANTS. 


nitely  known  at  this  temperature  ; therefore  “ it  is  more  scien- 
tific to  take  an  easily  verified  quantity  for  the  standard.” 

The  British  thermal  unit  now  used,  or  the  mechanical  equiv- 
alent of  the  heat  required  to  raise  the  temperature  of  one 
pound  of  water  one  degree  from  62  degrees  to  63  degrees  Fahr , 
is  that  of  Professor  Rowland’s  determination,  of  778  foot-pounds. 
This  temperature  is  also  that  of  the  new  British  standard  of 
weights  and  measures,  and  more  clearly  approaches  the  15 
degrees  Cent,  of  the  French  standard — a temperature  frequently 
used  and  readily  maintained  in  electrical  and  other  testing 

O 

work. 

The  only  objection  to  the  British  thermal  unit  used  in  connec- 
tion with  the  watt  is  that  this  heat-unit  is  not,  like  the  watt,  one 
of  the  units  based  on  the  scientific  standards  of  the  C.  G.  S. 
system.  Of  course,  such  a heat-unit  is  to  be  found  in  the 
calorie.  But  its  mechanical  equivalent  requires  that  the  most 
accurate  scientific  measurements  be  made  at  the  critical  tem- 
peratures for  water,  which  have  been  found  to  be  quite  impos- 
sible of  determination. 

(3)  PRESENT  USE  OF  THE  HEAT-UNIT  IN  STEAM  PUMPING  PLANTS. 

The  use  of  the  lieat-unit  in  this  connection  was  first  sug- 
gested* by  Dr.  Chas.  E.  Emery,  member  of  this  Society.  He 
proposed  that  the  duty  of  pumping  engines  should  be  based 
upon  the  foot-pounds  of  work  done  by  the  steam-pump  on  an 
expenditure  of  1,000,000  B.  T.  U.  in  the  steam  supplied  to  the 
plant. 

The  adoption  of  this  standard  was  the  logical  outcome  of  the 
old  one,  by  which  the  “ duty  ” was  rated  in  foot-pounds  per  hun- 
dred pounds  of  coal  burned  under  the  boiler.  The  ordinary 
specification  of  an  evaporation  of  10  pounds  of  water,  from  and 
at  212  degrees  Fahr.,  under  atmospheric  pressure,  would  result 
in  965,700  B.  T.  U.  supplied  to  the  whole  plant  per  hundred 
pounds  coal  burned  under  the  boiler.  Another  advance  was 
made  when  it  was  proposed  to  state  this  “ duty  ” on  the  basis  of 
the  1,000  pounds  of  feed  water  supplied  to  the  system,  rating 
the  heat  units  per  pound  of  water  evaporated  as  before,  from 
and  at  212  degrees  Fahr.  But  it  was  clear  that  the  efficiency  of 
the  boiler  should  have  nothing  to  do  with  the  “ duty  ” of  the 

* United  States  Centennial  Commission,  International  Exhibition,  Group  XX.  ; 
Vol.  VI.,  pages  21  and  115  of  the  Report  of  the  Judges. 


ON  EATING  ELECTRIC-POWER  PLANTS. 


pumping  engine.  It  was  therefore  hut  a slight  step,  and  one  in 
the  right  direction,  to  change  the  standard  to  that  of  1,000,000 
B.  T.  U.  in  the  steam  supplied  to  the  plant. 

Such  use  of  the  heat-unit  was  further  considered  before  this 
Society  in  the  “Report  of  Committee  on  a Standard  Method 
of  Conducting  Duty  Trials  of  Pumping  Engines.”  * The  adop- 
tion of  this  method  has  since  resulted  in  most  remarkable 
developments  in  this  branch  of  industry.  Unprecedented  econ- 
omies of  pumping  engines  have  been  brought  about.  Installa- 
tions and  contract  trials  upon  such  a basis  have  furnished 
valuable  engineering  precedent.  The  “duty”  of  pumping  en- 
gines can  be  so  closely  approximated  that  binding  contracts  are 
willingly  entered  into  by  the  builders.  Bids  are  advertised  for 
with  the  understanding  that  “ No  bids  will  be  considered  offer- 
ing less  duty  than  130,000,000  foot-pounds.” 

The  question  was  opened  afresh  and  the  cause  further  pro- 
moted by  Mr.  A.  F.  Hall  in  a recent  paper  f before  the  Society, 
in  which  he  writes  thus : 

“But  why  use  variable  quantities  as  coal  and  steam  for  units 
when  the  heat-unit  is  just  as  simple  to  obtain  and  use,  and  one 
which  requires  but  little  study  to  understand  ? ...  It  has  in 
it  the  elements  of  simplicity,  and  places  all  engines  upon  an  equal 
footing  for  comparison,  which  no  other  proposed  method  does.” 

(4)  PEESENT  WAY  OF  STATING  THE  PERFORMANCE  OF  ELECTEIC- 

POWEE  PLANTS. 

It  has  been  previously  noted,  in  the  abstract  of  advantages  of 
the  heat-unit  basis  over  the  present  ratings,  that  the  latter  are  : 
(a)  kilowatts  per  pound  of  coal ; (b)  kilowatts  per  cubic  feet  of 
gas  ; (c)  kilowatts  per  gallon  of  oil.  This  is  merely  to  inaugu- 
rate a method  of  stating  such  performance  for  electric  plants 
as  that  which  has  been  tried  and  found  wanting  for  pumping 
plants.  Neither  do  builders  of  engines  and  dynamos  care  to 
be  held  responsible  for  the  performance  of  the  heat-generating 
plant,  whether  it  be  a steam  boiler  or  a gas  producer. 

The  watt,  the  kilowatt,  or  the  watt-hour  basis  is  perfectly 

* 1 ransactions  of  the  American  Society  of  Mechanical  Engineers,  Vol.  XI.,  No. 
331 — “ Report  of  Committee  on  a Standard  Method  of  Conducting  Duty  Trials 
of  Primping  Engines.” 

f Transactions  of  the  American  Society  of  Mechanical  Engineers,  Vol.  XV., 
No.  584 — “ Heat-Units  and  Specifications  for  Pumping  Engines,”  by  A.  F.  Hall. 


8 


ON  EATING  ELECTRIC- POWER'  PLANTS. 


intelligible.  The  fuel  record  is  on  the  most  unsatisfactory  and 
unreliable  basis  that  could  have  been  selected  for  such  ratings. 
The  kind  of  coal  or  other  fuel  is  rarely  stated.  It  is  always  a 
variable  quantity.  Even  if  noted,  it  enables  one  to  form  only  a 
vague  notion  of  what  the  economic  rating  should  be  with  any 
other  kind  of  fuel.  Such  records  and  ratings  are  not  of  perma- 
nent value.  They  are  not  given  on  a rational  and  scientific 
basis  in  conformity  with  the  standards  in  electrical  and  other 
engineering  work.  It  is  proposed  to  substitute-  the  lieat-unit  as 
the  standard,  about  which  there  can  be  no  dispute,  unless  it  be 
as  to  the  kind  of  lieat-unit' which  should  be  chosen.  This  objec- 
tion has  been  considered  in  a preceding  section  with  regard  to 
the  merits  of  the  British  thermal  unit  and  the  calorie  of  the 
French  standard. 

(5)  THE  LOAD  FACTOR  IN  POWER-PLANT  RATINGS. 

When  the  output  of  the  plant  is  referred  to,  it  is  usually  in 
terms  of  its  load  factor.  Thus,  for  any  one  day’s  record  of  the 
performance  of  a good  electric  plant  it  would  be  at  present  ex- 
pressed as,  say,  200  watt-hours  per  pound  of  coal,  with  load 
factor  of  40.  From  the  very  definition  of  this  term  “ load 
factor  ” and  the  limitations  placed  upon  it,  it  is  apparent  that 
the  same  factor  may  be  obtained  in  quite  a number  of  different 
ways.  It  is  needless  to  point  out  that  there  will  be  a different 
coal  bill  for  each  and  every  set  of  conditions  giving  one  and  the 
same  load  factor. 

This  load  factor  expresses  the  rate  of  working.  Professor 
Unwin*  states  the  case  very  clearly  thus  : “ There  may  be  vari- 
ous load  factors  according  to  the  precise  fluctuation  considered. 
But  for  the  object  at  present  in  view,  the  consideration  of  the 
influence  of  variation  of  load  on  the  efficiency  of  steam  plant, 
the  load  factor  may  be  taken  to  be  the  ratio  of  the  area  of  a 
day’s  load  curve  to  the  area  of  a rectangle  enclosing  it.  It  is 
equally  the  ratio  of  the  average  load  during  the  day  to  the 
maximum  load  at  any  time  during  the  day.” 

Such  a load  factor,  therefore,  takes  no  consideration  of  the 
performance  under  a steady  load,  but  mainly  represents  the 
result  of  the  general  average  of  all  of  the  various  fluctuations 
during  any  given  time  interval,  as  one  day.  For  the  purpose  of 

* “ On  the  Development  and  Transmission  of  Power  fiom  Central  Stations,” 
hy  W.  C.  Unwin,  London,  1894. 


ON  EATING  ELECTRIC-POWER  PLANTS. 


9 


fchis  paper  it  cannot  be  considered  as  being  at  all  equivalent  to 
wliat  is  herein  termed  “ the  proportional  part  of  the  full  load,” 
under  which  condition  all  parts  of  the  system  may  be  consid- 
ered as  running  under  a constant  load.  It  is  the  performance 
under  the  latter  uniform  conditions  which  forms. the  basis  for 
comparison  of  ratings  by  the  British  thermal  unit  standard. 

(6)  PROPOSED  USE  OF  HEAT-UNITS  IN  ELECTRIC  PLANTS. 

It  is  proposed  to  state  the  performance  of  steam-power  elec- 
tric plants  in  kilowatts  per  1,000,000  B.  T.  U.  supplied  to  the 
steam  used  in  the  complete  plant. 

It  is  apparent  that  the  same  line  of  arguments,  pro  and  cow, 
is  quits  likely  to  arise  as  in  the  case  of  similar  ratings  for 
pumping  plants.  However,  such  a basis  is  equally  adapted  to 
meet  the  growing  requirements  for  some  standard  in  these  latter 
types  of  steam-power  plants. 

In  this  case,  moreover,  it  is  possible  to  start  upon  a right 
basis  from  the  beginning.  The  practice  of  rating  the  perform- 
ance upon  the  watt-hours  per  pound  of  coal  has  not  become  so 
rooted  that  it  cannot  be  changed  to  that  now  proposed.  The 
necessity  for  such  a standard  in  this  case  is  none  the  less  real, 
nor  is  its  final  adoption  less  probable  on  account  of  the  radi- 
cally different  conditions  under  which  these  two  types  of  power 
plants  are  regularly  operated  ; namely,  uniform  loads  in  pump- 
ing plants  and  extreme  and  often  rapidly-varying  loads  in  the 
electric  plants. 

As  long  as  electricity  continues  to  be  generated  by  machinery 
driven  by  heat-engines,  as  well  as  when  it  comes  to  be  generated 
from  coal  direct,  it  is  believed  that  there  is  no  better  standard 
than  that  already  adopted  in  steam  engineering.  The  unit  of 
measure  for  the  total  heat  put  into  the  system — namely,  the 
British  thermal  unit  —is  that  upon  which  the  output  in  kilowatts 
may  be  most  satisfactorily  based. 

(7)  DETERMINATION  OF  THE  HEAT  SUPPLIED  TO  THE  STEAM. 

The  method  of  procedure,  by  which  the  quantity  of  heat  sup- 
plied to  the  system  is  determined  from  the  feed-water  measure- 
ments, has  been  ably  set  forth  in  the  report  * of  the  Committee 

* Transactions  of  the  American  Society  of  Mechanical  Engineers,  Vol.  XL, 
No.  381 — “ Report  of  Committee  on  a Standard  Method  of  Conducting  Duty  Trials 
of  Pumping  Engines.” 


10 


ON  RATING  ELECT niC-POWER  PLANTS. 


on  Duty  Trials  of  Pumping  Engines.  It  is  equally  applicable  to 
the  case  of  the  electric  plant,  and  is  as  follows : 

“ Starting  with  a heat-unit  basis  of  computing  duty,  it  is  pro- 
posed to  make  the  computation  from  the  quantity  of  heat  sup- 
plied to  the  complete  plant ; using  not  only  that  supplied  to  the 
engine  cylinders,  but  that  supplied  to  all  the  necessary  parts  of 
the  engine,  such  as  the  steam-jackets,  the  donkey  feed-pump,  the 
independent  air-pump,  if  this  be  driven  with  steam,  and  any 
other  apparatus  using  steam  which  is  necessary  to  the  opera- 
tion of  the  engine. 

“In  contract  tests,  if  a steam-pump  be  used  for  the  boiler 
feed-pump,  the  quantity  of  heat  supplied  for  operating  this  ap- 
paratus is  to  be  included  in  the  total  quantity,  not  only  in  cases 
where  both  boiler  and  engine  are  supplied  by  one  party,  but 
also  where  the  boiler  is  furnished  by  a separate  contractor.  In 
this  connection  it  should  be  added  that  if  the  engine  contractor 
does  not  furnish  the  boiler  feed-pump,  he  should  be  permitted 
to  specify,  if  he  desires,  the  kind  of  feeding  apparatus  which 
shall  be  used  during  the  test. 

“ The  heat-unit  method  requires  that  the  actual  total  heat  of 
the  steam  shall  be  known,  and  for  this  purpose  allowance  will 
necessarily  be  made  for  any  moisture  or  superheat  contained  by 
the  steam  furnished  to  the  engine.” 

The  method  is  further  thus  specified  in  Mr.  A.  F.  Hall’s 
paper,  previously  referred  to  : 

“ Each  pound  of  water  fed  to  the  boiler  is  to  be  debited  with 
the  heat  required  to  raise  all  of  the  water  from  the  temperature 
it  has  at  its  entrance  to  the  boiler  to  that  corresponding  to  the 
boiler  pressure,  and  the  amount  of  heat  required  to  convert  97 
per  cent,  of  the  water  into  steam  of  boiler  pressure  from  the 
temperature  corresponding  to  this  pressure.” 

This  is  allowing  for  3 per  cent,  moisture.  The  amount  of 
moisture  in  any  case  would  be  determined  by  calorimetric 
measurements  of  the  quality  of  the  steam  supplied  by  the  boiler 
to  the  engine,  and  similarly  allowed  for. 

(8)  PERFORMANCE  OF  THE  BOILER  NOT  IN  EVIDENCE. 

This  is ’the  case  for  similar  reasons  to  those  which  have  ruled 
out  the  boiler  p erformance  in  determining  the  duty  of  pumping 
plants.  The  steam-generating  plant  and  the  steam  motive- 


ON  EATING  ELECTBIC-PwWER  PLANTS. 


11 


power  installation  form  two  elements  wholly  distinct  from  each 
other.  They  may  be,  in  some  cases  are,  quite  independent  of 
each  other. 

The  thermal  value  of  the  fuel  used  must  be  brought  into  the 
case  if  the  performance  of  the  plant  is  to  be  based  upon  the 
heat  supplied  to  the  boiler  furnace.  Such  a coal  basis  is  too 
uncertain,  and  is  the  least  desirable  one  at  present  from  which 
to  determine  the  heat  supplied. 

The  boiler  plant  is  the  only  portion  of  the  installation  in 
which  the  imman  element  has  a hand  in  the  economic  per- 
formance. 

It  is  most  desirable  to  eliminate  this  as  far  as  possible. 
Upon  the  skill  of  the  fireman,  or  upon  what  may  be  called  his 
personal  equation,  very  much  of  the  economy  and  efficiency  of 
the  boiler  will  depend.  If  mechanical  stokers  are  used,  even 
then  the  skill  of  the  attendant  is  quite  an  important  item. 

However,  at  this  stage  of  development,  builders  of  engines 
and  dynamos  do  not  care  to  be  responsible  for  the  performance 
of  the  boiler  plant,  which  is  often  furnished  and  installed  by 
entirely  different  parties.  They  wish  to  know  the  *•  duty,”  as 
in  the  case  of  pumping  engines,  regardless  of  the  boilers,  or 
irrespective  of  how  the  steam  is  supplied,  provided  it  is  com- 
mercially dry. 

(9)  DETERMINATION  OF  THE  WORK  DONE  BY  THE  ELECTRIC 
GENERATORS. 

This  reduces  simply  to  voltmeter  and  ammeter  readings  at 
the  switchboard.  These  are  to  be  further  corrected  for  any 
instrumental  errors.  The  load  during  trial  should  be  main- 
tained as  nearly  constant  as  practicable.  Latest  forms  of  re- 
cording instruments  make  it  quite  possible  to  obtain  accurate 
records  throughout  any  specified  period.  The  extreme  simplic- 
ity and  the  high  degree  of  accuracy  attainable,  in  the  case  of 
the  electric  plant,  should  be  strong  points  in  favor  of  adopting 
such  a standard  rating  as  here  proposed. 

Direct-connected  units  in  electric  plants  make  this  method 
as  feasible  and  as  practicable  as  in  direct-acting  pumping 
engines.  It  is  of  course  possible,  but  quite  improbable,  that 
steam-power  electric  plants  of  any  magnitude  will  be  belt- 
driven  in  the  future.  The  standard  performance  of  such  instal- 


12 


ON  HATING  ELECTBIC-POWER  PLANTS. 


lath  n involves  taking  into  account  the  small  percentage  of  loss 
aue  to  the  slipping  of  the  belt  or  rope  drive.  This  must  be 
stipulated  in  the  specifications  and  allowed  for  in  like  manner 
ro  the  percentage  of  slip  in  steam-pumping  engine  trials.  The 
cases  are  not  exactly  analogous,  but  they  may  be  similarly  con- 
sidered in  the  specifications  and  determined  in  the  contract 
trial. 


